Recent development in mobile communications and information and electronic industry has created continuously increasing demand for the secondary batteries which are light-weighted, but provide high capacity. However, when overcharged or short-circuited, the secondary batteries can generate excessive heat, which can possibly lead into fire or explosion. When over-discharged below normal voltage range, capacity rapidly decreases and the batteries can enter into such a state that it is not possible to use the batteries.
For these reasons, batteries have always been equipped with safety devices such as protection circuits and PTC since they have been first developed. However, the protection circuits and PTC are expensive and occupy considerable volume, leading to undesirably increased battery price, volume and weight. Accordingly, development of a battery is necessary, which can reduce production cost without protection circuits and PTC, and also can increase capacity of the battery.
The related technologies solve safety issues by incorporating organic or inorganic additives into the non-aqueous electrolyte solution or by altering the outer structure of the battery in case of battery overcharge or short-circuit. However, when the battery is over-discharged below a rated voltage, the capacity can have been rapidly declined to the level that it is difficult to charge or discharge further by the time the battery is charged again.
The general secondary lithium battery developed so far is configured in such a structure that the negative electrode discharge is limited and thus is cutoff in case of over-discharge. Specifically, at initial charging, the non-aqueous secondary lithium battery is formed with a solid electrolyte interface (SEI) film on the surface of the negative electrode, and at this time, a large amount of lithium ions released from the positive electrode is used. Accordingly, the amount of Li participating in charging and discharging is declined. When the battery is subjected to over-discharge with this reduced Li amount, the activated Li sites of the positive electrode are not completely filled and this leads into the phenomenon that the voltage of the positive electrode is kept from declining to below a predetermined voltage. Accordingly, discharging is ended by the negative electrode.
Meanwhile, the reason for rapid capacity decline after over-discharge is as follows. The battery voltage is defined by the voltage difference between positive electrode and negative electrode, and if a battery is continuously discharged at low current even after the voltage drop below a general threshold voltage, the voltage of the positive electrode is kept from further drop due to Li ion consumption at the negative electrode. As a result, while the voltage of the positive electrode is on a gradual decline, the voltage of the negative electrode is on a relatively rapid rise, reaching 3.6V at which the copper foil used as a current collector of the negative electrode is oxidized. In the example described above, the copper foil is melt into copper ion state, contaminating the electrolyte and adhered to the surface of the negative electrode. After that, it is not possible to use the negative electrode active material. As described, when the copper foil is subjected to oxidation, capacity is rapidly reduced after over-discharge so that it is not possible to use the battery anymore.
Accordingly, development of a battery is necessary, which limits discharge by the positive electrode and thus prevents considerable reduction of the battery capacity, and development of a new method is also necessary, which can produce such positive electrode-limited battery.